1. Field of the Invention
The present invention relates to a method of controlling an internal combustion engine when the engine is operating in an engine braking mode.
2. Background Art
The heavy duty engine business is extremely competitive. Increasing demands are being placed on engine manufacturers to design and build engines that provide better engine performance, improved reliability, and greater durability. One feature on some existing engines is engine braking. Engine brakes, or retarders, are used to assist and supplement wheel brakes in slowing heavy vehicles. Engine brakes are desirable because they help alleviate wheel brake overheating. Known engine compression brakes convert an internal combustion engine from a power-generating unit into a power-consuming air compressor by prematurely opening an exhaust valve of at least one cylinder to dissipate power.
The control system for a compression brake mechanism on an engine with a traditional fixed geometry turbocharger utilizes an engine control module algorithm that among other things, senses throttle and a dash mounted multi-position switch. Using the switch, the vehicle operator sets and limits the number of cylinders in brake mode and therefore the general brake power level whenever throttle goes to zero fueling. Typically, the engine control module activates six, four, or two cylinders. The engine then operates along a high, medium, or low brake power curve as engine speed varies due to grade changes. Such a control system provides relatively stable control for vehicle speed as varying grades are encountered because as speed increases or decreases, brake power increases or decreases, respectively. The vehicle operator selects a general brake level with the multi-position switch and selects a transmission gear based on the operator's judgment of traction conditions, desired mean vehicle speed down the grade, and desired speed variability.
In some applications, tighter vehicle speed control is provided by closed loop vehicle speed control. The number of cylinders in braking mode is determined by the magnitude of the vehicle speed error from a driver determined speed set point. Typically, this implementation is done in a step wise fashion with the number of braking cylinders increasing as the speed error increases. Such a technique provides fuel economy benefits by delaying brake power increases until some acceptable over speed occurs.
However, although the above techniques have been utilized in some applications that have been commercially successful, there are sources of brake power variability over which the driver has no control. For example, the braking power curves vary as altitude and ambient temperature change. That is, brake power decreases as altitude increases. Further, colder inlet temperatures result in increases brake power. Other sources of brake power variability include turbocharger part-to-part variability, accumulated wear and damage, manifold leaks, etc. An existing engine compression braking apparatus is shown in U.S. Pat. No. 6,148,793.
For the foregoing reasons, there is a need for a method of controlling an internal combustion engine when the engine is operating in an engine braking mode that provides improved control over braking power.